I set lang-py as the code language used for syntax highlighting for the python-2.7 tag. It was previously set to none (the default). This should fix the problem for any other questions tagged python-2.7 but not python.

The reason is that the version-specific tags need to be explicitly added by the admins/mods in order to pick up syntax highlighting for the base language, in cases where the base tag (or any other tag that defaults to the same language) is not included. I complained about this for SQL Server 2008 R2 and SQL Server 2012:

The general solution going forward seems to be to add explicit version tags to the syntax highlighter as they come out, rather than solve for all potential versions using regex or wildcards. Which I think is more scalable but seems to be opposed.